precomp decorator support

nb.precomp decorator can be used to wrap BudgetItem.calc methods with
functions that only need to be called once per budget run.

    nb.precomp(precomp_foo)
    def calc(self):
        ...

Precomp functions are all executed by the update() method (after attribute
update), and execution state is cached at the Budget level, to prevent
re-calculating the same functions multiple times per update.

The BudgetItem.update() method is no longer expected to be overloaded by
the user.

This provides a speed-up of about 50% for the Aplus model, but much less
for the CE models which are limited by other noise calculations.